U.S. Pat. No. 4,774,626 (corresponding to Japanese Patent Application Laid-Open Publication No. 19527/1988) discloses a capacitance-type pressure sensor which includes a base substrate provided thereon with a main capacitive electrode and a diaphragm substrate provided thereon with a counter electrode in a manner to be opposite to the main capacitive electrode and is so constructed that the base substrate and diaphragm substrate are joined at an outer periphery thereof to each other through a layer of sealing material, as well as a pressure sensor module including such a capacitance-type pressure sensor. U.S. Pat. No. 4,888,662 likewise discloses a pressure sensor module including a capacitance-type pressure sensor. As seen in each of the U.S. patents, fluid of which a pressure is to be measured is contacted with a rear surface of the diaphragm substrate of the pressure sensor.
U.S. Pat. Nos. 4,329,732, 4,735,098, 4,680,971 and 4,425,799 each disclose a capacitance-type pressure sensor wherein a base substrate is provided thereon with a reference capacitive electrode separately from a main capacitive electrode.
FIG. 19(A) schematically shows a conventional capacitance-type pressure sensor which includes a base electrode provided thereon with a main capacitive electrode and a reference capacitive electrode. The conventional capacitance-type pressure sensor includes a base substrate 31, which is formed thereon with a main capacitive electrode 32 and a reference capacitive electrode 33 in a manner to be spaced from each other at predetermined intervals. Also, it includes a diaphragm substrate 34, which is formed thereon with a diaphragm electrode 35 in a manner to be opposite to the main and reference capacitive electrodes 32 and 33. In the conventional capacitance-type pressure sensor thus constructed, external application of a pressure P of fluid which is to be measured to the diaphragm substrate 34 leads to deflection of the diaphragm substrate 34, resulting in a distance or interval between the main capacitive electrode 32 and the diaphragm electrode 35 being varied from D0 to D1, so that a capacitance between the main capacitive electrode 32 and the diaphragm electrode 35 may be varied, as shown in FIG. 19(B). Such a capacitance-type pressure sensor permits the pressure P to be measured or calculated on the basis of both a capacitance varied between the main capacitive electrode 32 and the diaphragm electrode 35 and a reference capacitance between the reference capacitive electrode 33 and the diaphragm electrode 35.
For the purpose of measuring the pressure P at increased sensitivity, the reference capacitance between the reference capacitive electrode 33 and the diaphragm electrode 35 is desirably kept from being varied due to application of any pressure to the capacitance-type pressure sensor. Nevertheless, such application of the pressure P to the diaphragm substrate 34 as shown in FIG. 19(B) causes the distance between the reference capacitive electrode 33 and the diaphragm electrode 35 to be slightly varied from D0 to D2, leading to a variation in reference capacitance between the reference capacitive electrode 33 and the diaphragm electrode 35. Thus, the conventional capacitance-type pressure sensor fails to measure a pressure at increased sensitivity.